CHAPTER 2
LITERATURE REVIEW
2.1
Precast Definition Definition
Precast concrete can be defined as concrete, which is cast in some location
other than its final position in a structure (Phillips and Sheppard, 1980). The precast
concrete components may be prestressed pilings, hollow core slabs, planks, double
tee slabs, beams, columns, staircases, walls, facades, railway sleepers and others
(Eastern, 2004). Precast concrete components are reinforced either with mild steel
reinforcing bars or with prestressing strands. When prestressing is used in the
production of precast components, pretensioning is usually applied where long
strands are tensioned before pouring the concrete in long lines in the precasting
operation. One of the important precast concepts is all the precast elements must be
connected for integrity and stability (Hartland, 1975). Therefore, many research on
precast connection have been carried out recently.
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2.2
Connections in Precast Concrete Constructions
Generally, the precast concrete connections are classifying by the relationship
of moment-rotation. With this relationship, connections can be classified into 3 types
as bellowed:
a)
Rigid connection
b)
Semi-rigid connection
c)
Simple connection
Connections determine how the forces (moment and axial load) being
transferred between two structural members. The rigid connection and simple
connection are the idealized assumptions that indicating full-moment transfer and
zero-moment transfer; the semi-rigid connection is in actual condition that stands in
between. Figure 2.1 shows the difference between the effects of connection types to
the force distribution in a structure.
Figure 2.1 : Moment distribution in various types of connections
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Connection design is one of the most important considerations for the
successful construction of precast reinforced concrete structures. The detailing and
structural’s behaviour of the connection affect the strength, stability, and
constructibility as well as the load redistribution of the building under loads (Loo and
Yao, 1995). The many different types of connections used by designers makes it
difficult to generalize on rules and guidance notes, particularly because different
methods of making the same type of connection are practiced by engineers.
Normally, the connections are used to connect between:
a)
Column or wall to the foundation
b)
Column to column
c)
Beam-to-column or wall
d)
Beam to beam
e)
Slab and /or staircase to beam or wall
f)
Slab to slab
g)
Structural steelwork, insitu concrete, timber and masonry to precast
concrete components.
Although there are so much connections that used to connect precast element,
but my study will focusing on the beam-to-column connections only. Therefore, only
beam-to-column connections will be discussed deeply in this study.
2.2.1
Rigid Beam-To-Column Connections
Rigid connections are used to assure sufficient stability of portal frames.
These connections are also used to limit the height of structural components such as
beams without loss of stiffness. Rigid connections are, in general, connections
between beams and columns and also between slabs and walls. These connections
are also able to prevent progressive collapse of multi-storey structures (Bruggeling
and Huyghe, 1991). In most cases, connections with fully fixed are very hard to
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achieve. Therefore, connections that able to resist more than 90% of fixed end
moment can be assuming as rigid connections (Hanif Alias, 2002).
Although there are many rigid connections use in precast concrete
construction, this study is more concentric on the pinned connections. Therefore, it
will not be discuss into detail.
2.2.2
Semi-Rigid Beam-To-Column Connections
Semi-rigid connections have strength in between simple connections and
rigid connections. Therefore, these connections are transferring some of the moments,
axial forces and shear forces from the beam to support.
2.2.3
Simple Beam-To-Column Connections
Pinned connections transfer purely shear forces, both for (dominant) gravity
and uplifting forces. Those connections with moment resisting smaller than 50kN.m
are considered as pinned connection. Pinned connections lend themselves to simple
detailing and construction, and may be formed in the simplest manner by element
bearing. To increase bearing capacity and reduce localized spalling, steel inserts are
frequently used by anchoring plates (or rolled structural sections) into the connecting
elements. The connection is completed by surrounding the steel inserts with
cementitious grout for the fire and durability protection (Elliott, 1996).
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In the simply supported beam-to-column, the column has to meet the
following requirements:
a)
Allow for rotation of the beam above the column.
b)
Allow for horizontal movements of beam ever with the initiation of
small horizontal forces.
c)
Transfer the support forces safely.
d)
The supporting material must bridge the tolerances between these
elements and their supports without considerably reducing the loadcarrying capacity of the connection when the placing of the beam
during construction stage (Bruggeling and Huyghe, 1991).
2.2.3.1 Beam on Top of a Column
This connection is shown in Figure 2.2. The connection can transfer the
vertical loads and some horizontal forces from the beam to the top of the column.
Normally, a vertical bar protrudes from the top of the column. This bar corresponds
to a vertical hole in the beam end. To allow free movement between the beam and
the column the hole is filled, after erection of the beam, with mastic or mortar.
Because of the tolerances in the beam length, the placing of the column and the
location of the protruding bar in the top of the column, problems may arise during
the placing of the beams. Therefore, the hole in the beam should allow for tolerances.
To avoid cracking of the beam end as a result of unforeseen horizontal forces (during
erection) horizontal reinforcement must be provided around the hole.
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Figure 2.2 : Beam on top of a column (Elliot, 1996)
2.2.3.2 Beam-To-Column with Haunched Column
This design involves only the provision of an adequate bearing surface at the
level of the soffit of the beam as shown as Figure 2.3. This type of connection is not
widespread in multi-storey frames because of the increased dimension of the column
at each floor level. The detail may be used in low rise unbraced structures where the
increased column is beneficial in cantilever action or in situations where the
magnitude of the beam end reaction cannot practically or structurally be
accommodated in a shallow corbel or steel insert.
A bearing medium between the concrete surfaces of at least 10mm thickness
is highly recommended to ensure a uniform bearing pressure, to avoid eccentricity of
load, to prevent local spalling, to accommodate tolerances in very long columns and
to allow beam rotations to take place. The bearing pad may consist of a neoprene, or
be a composite construction of two thin (3mm thick) steel plates with neoprene
(10mm) sandwiched between. In all cases the edge of the bearing plate should not
extend beyond a point connecting a 45 line to the edge of the top steel in the
column. Some precasters provide a 25 x 25mm chamfer to ensure that the bearing
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pad does not extend to the edge of the column. Also the bearing pad should not
project beyond the end of the beam where there is a clearance gap of 10 to 25mm.
Figure 2.3 : Beam-to-column connection with haunched column (Elliot, 1996)
2.2.3.3 Beam-To-Column with Corbel and Cast-In Dowel
A corbel is a short cantilever projection from the face of a column (or wall)
which supports a load bearing component on its upper horizontal ledge. As in any
type of the frame it is necessary to minimize the structural zone by containing the
connection within the overall depth of the beam. For reasons of appearance this calls
for ‘shallow’ corbels, which are designed as a short cantilever. Care is needed to
ensure correct lapping and anchorage of reinforcement adjacent to contact surfaces,
with full regard for fixing tolerances.
The usual beam connection is a single vertical dowel which is a waiting bar
cast into the corbel. The dowel diameter is between 16 and 25mm typically, and the
respective dowel hole is 35 to 50mm diameter. Non-shrink grout is poured into the
hole from the top. If a dowelled connection is made between the beam and corbel,
horizontal hair-pin bars (typically R 8 diameter at 50mm centers) are used to prevent
bursting of the thin covering to the dowel hole and projecting dowel bar. Beam-tocolumn connection with corbel and cast-in dowel is shown in Figure 2.4.
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Figure 2.4 : Beam-to-column connection with corbel and cast-in dowel
(Bruggeling and Huyghe, 1991)
2.2.3.4 Welded Beam-To-Column Connection
This connection is shown in Figure 2.5. The tensile forces in the connection
should be transferred to the column as well as to the beam with sufficient safety. The
steel parts of the connection should, therefore, be anchored into the concrete
structure by means of anchors (bars) welded to these steel parts. The strength of the
welds should allow for the transfer of forces.
Figure 2.5 : Welded beam-to-column connection (Bruggeling and Huyghe, 1991)
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2.2.3.5 Beam-To-Column with Mushroomlike Corbels
The connection is shown in Figure 2.6. In this case the corbel is threedimensional. Instead of an oblique strut under compression of the two-dimensional
case, a vault under compression can now be assumed in the engineering model. The
angle α of this vault in relation to a horizontal plane is limited to 40 to 45. In the
top of this corbel the horizontal plane is now loaded in tension in two directions. The
tensile forces in this plane are equilibrium with the loads on the corbel and the
compressive forces in the vault. In the bottom of the corbel another plane is loaded in
compression.
Figure 2.6 : Beam-to-column with mushroomlike corbels (Bruggeling and Huyghe,
1991)
2.2.3.6 Beam-To-Column with Steel Corbels
The connection is shown in Figure 2.7. This connection is one of the
commonest form of the beam-to-column connection. The structural mechanism is
based on static strength, stiffness, load transfer into the connecting components,
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temporary stability and structural integrity. The basis of connection design is as
follows:
a)
At A – to transfer the shear force carried by reinforced concrete at the
ends of the beams into the connection. This usually takes the form of
shear transfer from a combination of vertical shear links and/or bent
up bars into a prefabricated steel section, called a shear box. The end
of the beam is pocketed or recessed.
b)
At B – to ensure adequate shear capacity in the plane of the physical
discontinuity between beam and column taking into consideration
ultimate and working values of static strength, cracking, durability
and fire resistance. This usually takes the form of either a projecting
steel section or a gusseted angle or T-cleat bolted into anchored
sockets.
c)
At C – to transfer the compressive loads into the rc column. The
effects of horizontal bursting forces, both above and below the
connection in the case of eccentrically loaded columns, are taken care
of using closely spaced links. Column anchorages are generally either
fully anchored cast-in-sockets or steel box or H-section cast-in-inserts.
Figure 2.7 : Beam-to-column with steel corbels (Elliot, 1996)
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2.3
Conclusion
There are many types of pin connections which have been discussed in the
previous section. This indicates that pin connections are applicable in precast
construction. Among all the pin connections, beam-to-column connection with corbel
and cast-in dowel has been selected for this study. It is commonly use in Malaysia
precast industry, because of its simple detailing and construction